The present invention relates in general to semiconductor devices and in particular to a diffusion barrier structure used in a semiconductor device for suppressing reaction between a wiring electrode and a semiconductor substrate.
In conventional semiconductor devices, an aluminum-based wiring electrode made of aluminum or aluminum-silicon alloy is contacted with a silicon substrate directly. In such a structure, there is a problem in that aluminum wiring electrode reacts with silicon the substrate when the semiconductor device is held at a relatively high temperature such as 450.degree. C. during the manufacturing process of the semiconductor device. When such a reaction occurs, silicon is dissolved into the aluminum electrode and there is a tendency that silicon thus dissolved into the electrode is precipitated at a boundary between the electrode and the substrate as an epitaxial phase. When the epitaxial silicon phase is grown at the boundary, the resistivity across the contact is increased. Further aluminum in the electrode, too, is diffused into the silicon substrate from the electrode and forms an alloy spike structure in the substrate which is a sharp-pointed spike-like region enriched in aluminum. The alloy spike extends into the interior of the substrate from the boundary between the electrode and substrate and causes unwanted short-circuit conduction at a junction of the semiconductor device in the substrate particularly when the junction is formed in an extremely shallow region of the substrate. When such an unwanted conduction occurs, the semiconductor device no longer operates properly.
In order to eliminate these problems associated with the reaction between the silicon substrate and the aluminum or aluminum alloy wiring electrode, use of a diffusion barrier structure is proposed in which the reaction between the silicon substrate and the electrode is blocked by a barrier layer provided between the electrode and the substrate. This barrier layer is generally called a barrier metal layer although the material for the barrier layer is not limited to metals. In the present specification, the barrier layer will be referred to as barrier metal layer according to general practice. Such a barrier metal layer prevents the diffusion of silicon and aluminum passing therethrough. The barrier metal layer may be a film of titanium nitride (TiN) or a titanium tungstenite (TiW) and is deposited on the substrate by reactive sputtering, evaporation, chemical vapor deposition (CVD) and the like before the deposition of the wiring electrode.
It has been found, however, that the conventional barrier metal layer cannot prevent the diffusion of silicon and aluminum effectively particularly when a thin barrier metal layer is used. The reason of this unsatisfactory result is generally attributed to the microstructure of the barrier metal itself. More specifically, the barrier metal layer deposited on the substrate generally has a columnar microstructure in which the grain of the material is elongated generally perpendicularly to the plane of the substrate, and the grain boundary between the elongated grains in the barrier/ metal layer provides a diffusion path for silicon and aluminum. As the grain boundary generally extends through the barrier metal layer from one side to the other, it is reasonable to assume that a substantial number of silicon and aluminum atoms pass through the barrier metal layer relative ease by diffusion.
In order to minimize the diffusion across the barrier layer, one has to use a relatively thick barrier metal layer such as 1500 .ANG. or more. However, such a thick barrier metal layer is disadvantageous due to the increased stress in the barrier metal layer. Further, the resistance of the diffusion barrier structure is increased with increasing thickness of the barrier metal layer.
Alternatively, it is proposed to oxygenate the barrier metal layer thus formed in order to prevent the diffusion of aluminum and silicon through the grain boundary. In doing so, it is assumed that the oxygen penetrates into the barrier metal through the grain boundary and the diffusion path is blocked by oxygen.
Experimental result did show such a decrease in the diffusion of elements across the barrier metal layer with oxygenation, but at the same time it was shown that the resistivity of the barrier metal layer increases with increasing oxygen content in the barrier metal layer (Stimmel J. B. and Mehrotra B. N. "Effects of Oxygen on Reactively Sputtered TiN Films". In: Tungsten and Other Refractory Metals for VLSI Applications III, V. A. Wells ed., pp. 375-382, Materials Research Society, 1988). Such an observation suggests the possibility that oxygen does not only exist at the grain boundary but also exists at the surface of the barrier metal layer in a form of oxide. In other words, the surface of the barrier metal layer is oxidized as a result of the oxygenation. In order to, prevent the diffusion of silicon and aluminum across the barrier metal layer effectively by oxygenation, a substantial amount of oxygen has to be introduced into the barrier metal layer particularly when the thickness of the barrier metal layer is thin. However, such an oxygenation increases the resistivity of the barrier metal layer as already described. Therefore, the oxygenation of the conventional barrier metal layer in the diffusion barrier structure is not preferable from the view point of the increased resistivity across the diffusion barrier structure.